The specialized olfactory receptor neurons of the olfactory nerve are located in the olfactory mucosa of the upper parts of the nasal cavity. Olfactory receptor neurons continue to be born throughout life.The olfactory nerve is the shortest of the twelve cranial nerves and only one of two cranial nerves (the other being the optic nerve) that do not join with the brainstem.

Function:

Carry sensations of smell from the nasal mucosa to the olfactory bulb, from there through the olfactory tracts and roots especially the lateral root to the peri amygdaloid and pre piriform areas of the cortex, the uncus and the hippocampal gyrus. The sense of smell (olfaction) arises from the stimulation of olfactory (or odorant) receptors by small molecules of different spatial, chemical, and electrical properties that pass over the nasal epithelium in the nasal cavity during inhalation.

Testing:

Small bottles of very familiar odour essences like that of coffee, almonds, chocolates, peppermint etc are used. Patient should compress one nostril and take two sniffs of a bottle and asked to identify wether he could

i.Smell anything,

ii.Identify the smell.

The patient is to repeat the procedure for the other nostril and responses are noted and compared. After some interval the test is repeated using two more odours.

Interpretation:

a.Recognize and name the odours quickly(usually women),

b.Recognize but cannot name them(usually men),

c.Detect a smell but cannot recognize or name them,

d.Each odour smell the same but unpleasant or distorted(parosmia),

e.Detect no/reduced smell in one nostril or both( anosmia),

f.Vague and variable responses.

a,b, and c should be accepted as normal. Others provide reasons for differential diagnosis for the reasons for altered smell sensation.